A visa is required for entry into Kenya if travelling from most countries including the UK, USA, Canada and Europe The cost is £30GBP or USA $50. You can apply for the visa by post to the Kenya High Commission in London (for British citizens)  . Payment needs to be made with either a Postal Order or Bankers Draft. It normalley takes about 5 working days for applications to be processed. Visas can also be obtained on arrival in Kenya but payment will need to be made in cash with either UK Pounds, Dollars or Euros.

Application forms can be downloaded from the Kenya High Commission web site or Kenya Airways  here:

http://www.kenya-airways.com - Visa Info link

http://www.kenyahighcommission.net - Visa links - links can be found on the lower left of the page

It is advisable to purchase your visa prior to arrival,  as sometimes there can be very long queues of people waiting to purchase a visa at the airports.

The form you download from the internet is the same one they provide at the airport terminal in Kenya so even if  you delay getting your visa until arrival, it's still worth filling it out beforhand so you jump the line of other people at the terminal who are busy filling the form in.